1. Field of the Invention
The present invention relates to an air assist device of an engine.
2. Description of the Related Art
To atomize the fuel injected from a fuel injector, use has conventionally been made of an air assist device which arranges the fuel injector in the intake passage downstream of the throttle valve, branches off an assist air passage from the intake passage upstream of the throttle valve, and blows the assist air ejected from an assist air ejection port of the assist air passage against the fuel injected from the fuel injector. As such an air assist device, there is known an air assist device wherein a narrow rectangularly shaped air intake port is formed in the circumferential direction in the inner wall of the engine intake passage so as to take in the assist air to be supplied to the fuel injected from the fuel injector and the area of the opening of a rectangular opening portion of the air intake port is controlled by the outer peripheral end face of the throttle valve (see Japanese Unexamined Patent Publication (Kokai) No. 6-213107).
In this air assist device, however, there is the problem that it is not possible to obtain the optimal amount of assist air for the operating state of the engine. That is, referring to FIGS. 10A and 10B showing the positional relationship between the throttle valve Y when at the idling position and the rectangularly shaped air intake port Z formed in the inner wall of the intake passage X, since a certain amount of assist air is required for idling operation, the part Za of the air intake port Z has to be made to open at the upstream side of the throttle valve Y. FIG. 10A shows the optimal area Za of the opening at this time.
On the other hand, a large amount of assist air becomes necessary as the throttle valve Y opens, the area Za of the opening of the air intake port opening at the upstream side of the throttle valve Y is made to gradually increase as the throttle valve Y opens. Next, when the throttle valve Y further opens, the entire air intake port Z is made to open to the upstream side of the throttle valve Y. At this time, however, it is not possible to secure a sufficient amount of assist air with the area of the opening of the air intake port Z shown in FIG. 10A.
If, therefore, the length of the air intake port Z in the circumferential direction is increase as shown in FIG. 10B so that a sufficient amount of assist air is obtained when all of the air intake port Z opens to the upstream side of the throttle valve Y, the bottom edge portions Zb of the two ends of the air intake port Z will open to the downstream side of the throttle valve Y. If the air intake port Z opens to the downstream side of the throttle valve Y, the air flowing to the air intake port Z due to the large negative pressure caused at the downstream side of the throttle valve Y will be drawn to the downstream side of the throttle valve Y, so almost no assist air will flow. Therefore, the air intake port Z cannot be made to open to the downstream side of the throttle valve Y.
If, accordingly, the position of the air intake port Z shown in FIG. 10B is shifted to the upstream side so that the air intake port Z does not open to the downstream side of the throttle valve Y, the area Za of the opening of the air intake port opening to the upstream side of the throttle valve Y during idling operation will end up becoming extremely large compared with the optimal value shown in FIG. 10A.
In this way, if the opening portion of the air intake port Z is formed in a rectangular shape, it will not be possible to secure the optimal amount of assist air for the engine operating state no matter how the dimensions, shape, or position of the air intake port Z are changed.